1. Field of the Invention
The present invention relates to a method for agglomerating and dewatering polymeric materials and, in particular, elastomeric materials, as part of the overall drying process which occurs subsequent to polymerization. More specifically, the invention pertains to a method wherein elastomeric material having a relatively high water content, approximately 35 to 60%, is fed into the tight nip opening formed by a pair of counterrotating pressing rolls in a rubber crumb compactor apparatus and is agglomerated and dewatered therein. The elastomeric feed is presented to the compactor apparatus in the form of discrete fines and particles or as continuous sheets. The agglomerated product produced by the compactor apparatus from discrete fines and small particle feed is in the form of compacted, larger-sized particulates and, in the case of large particle or sheet feed, is in the form of continuous sheets. The water content of the feed material is reduced to around 10 to 30% by the rubber cumb compactor apparatus with a relative reduction in water content in the range of 30 to 80%.
In the production of synthetic elastomeric materials, such as butyl rubber, chlorinated butyl rubber, brominated butyl rubber, EPDM elastomers SBR, BR, etc., and the like, the product obtained from the polymerization process is in the form of an aqueous slurry. To finish the product, rubber particles which are essentially moisture free are prepared by a series of conventional drying steps.
A typical drying process first employs a vacuum drum, or vibrating or fixed screen, which receives the initial slurry containing only about 3 to 5 wt. % rubber, the balance of the weight being water. There is no significant particle size increase on the screens or in the vacuum drum and the rubber crumb which is produced thereby contains 35 to 60% by weight water. Further drying is then achieved by treating the material in a first dewatering extruder. This step typically provides rubber crumb particles containing from about 5% to 16% by weight water. The capacity of the first dewatering extruder is profoundly affected by crumb size and feed water content. More particularly, the capacity of the dewatering extruder is reduced dramatically with small feed crumb size and/or high feed water content.
It has been found, for instance, that expellers one particular type of dewatering extruder, encounter problems handling certain types of elastomeric grades. If the elastomer is such that it extrudes very easily, it is often difficult to input the work necessary to dewater the rubber. Additionally, the expeller drainage slots, formed by barrel bars, often plug with rubber and, due to low pressure in the expeller, the machine is unable to push the plugs out of the way to allow water to excape. Consequently, rubber rates are severely limited and the elastomeric material discharged often has an unacceptably high moisture content, around 15 to 17%. If the feed rate is increased in an effort to increase pressure in the expeller, the expeller barrel then fills with feed material and the material backs into the expeller hopper, causing the expeller to plug. The foregoing deficiencies and emphasized in cases of small feed crumb size, and/or high feed moisture content, resulting in unstable extrusion operation.
Thus, the need exists for a method to press free water from the inlet crumb and to form larger crumb by agglomerating small crumb upstream of the expeller. Expeller line rates and extruder plugging would then be improved and the feed section and pressing sections of the expeller would then be unloaded.
The subject method of dewatering and agglomerating polymeric materials utilizes a rubber crumb compactor apparatus having counterrotating rolls with a tight nip opening to compact the feed rubber crumb prior to the step of feeding the rubber cumb to the first dewatering extruder, to agglomerate the crumb into larger pieces or continuous mats, and to reduce the water content of the feed crumb to approximately 10 to 30% by weight water. Thus, the relative reduction in water content of the rubber crumb prior to being fed to the first dewatering extruder is in the range of a 30 to 80% reduction.
The method of the present invention greatly enhances the capacity of the first dewatering extruder. Indeed, in some instances, the first dewatering extruder can be replaced with the instant process and eliminated entirely. Furthermore, the effective dewatering achieved by compaction according to the present process, which utilizes principles of simple compression and simple shear, results in lower energy requirements than for a dewatering extruder, which transfers mechanical work into the rubber mass by the less efficient viscous shear mechanism.
Hence, the present invention achieves more efficient primary dewatering in a rubber emulsion process in a single squeezing step within a very short period of time. The invention results in agglomeration of the inlet crumb, regardless of size, with a minimum of doubling of the average particle size and, in most cases, easily forming continuous mats. The process results in a significant increase in the dewatering capacity of the first dewatering extruder by mitigating the limitations caused by small feed crumb particles and high moisture content. Moreover, the process allows for an increase in the capacity of the overall rubber dewatering-drying extruder process by decreasing the feed moisture content by a relative reduction of 30 to 80% by current methods. The compactor of the instant method may even replace the first dewatering extruder or the vacuum drum in some finishing operation scenarios due to its superior dewatering and agglomerating capability and lower horsepower requirements.
The instant method can be utilized in any rubber slurry or emulsion drying or devolatilizing process in which water or a non-solvent must be removed from the rubber particles. The invention is particularly effective in improving the operation of a dewatering extruder or similar mechanical device when the water or the non-solvent in the feed is greater than 15% weight water. The invention is able to augment or to replace the existing dewatering extruder or similar mechanical device based on its more effective performance and lower energy requirements.
2. Description of the Prior Art
As discussed previously herein, the conventional finishing process for water slurries of solid elastomers involves, in part, fixed or vibrating screens or vacuum drums prior to treating the material in a dewatering extruder. Representative disclosures of methods for removing moisture from elastomers are disclosed in U.S. Pat. No. 3,222,797, issued Dec. 14, 1965 to Zies; U.S. Pat. No. 3,240,746, issued Mar. 15, 1966 to Davis; and U.S. Pat. No. 4,508,592, issued Apr. 2, 1985 to Kowalski.
U.S. Pat. No. 3,222,797 is directed to a method for preliminary dewatering and finish drying of a variety of polymers, including elastomers, in an extruder device. The process involves transferring the polymer through an extruder so that both temperature and pressure are progressively increased.
U.S. Pat. No. 3,240,746 describes a process for removing water from solid elastomers wherein the elastomer crumb is supplied to a shaker screen and thence to a water expeller or screw press.
U.S. Pat. No. 4,508,592 teaches an elastomer extrusion drying process characterized by injecting gas into the compression zone of an extruder to provide improved efficiency in the explosion drying process.
A method for mechanically removing physically combined water from solid polymers is disclosed in U.S. Pat. No. 2,771,689, issued Nov. 27, 1956 to Bettes, Jr. The method comprises feeding a water saturated polymer into the nip of a press including a pair of parallel metal rollers. One roll is maintained cool and the other is maintained hot such that, when the rolls are counterrotated, the substantially water free polymer adheres to the hot roll and the water collects on the cool roll. The polymer is scraped from the surface of the hot roll and deposited on a conveyor by means of a blade, while water is removed from the cool roll by a wiper blade. The process and means for practicing the process are relatively complex and require extraneous steps, such as scraping the roll surfaces and the belt upon which the polymer is deposited.
None of the prior art methods for dewatering and drying polymeric materials teaches or suggests a simple and effective method for agglomerating and dewatering elastomeric materials which is capable of accommodating a variety of processing conditions and achieving significant compaction and dewatering results.
Various methods and devices are known in the prior art for removing a liquid phase from various solids. For example, U.S. Pat. No. 2,798,424 issued July 9, 1957 to Smith et al discloses a filter for dewatering sewage sludge comprising a wire screen in the form of a fluted cylinder which meshes with a similarly configured fluted drum. The two members are rotated in unison and partially dewatered sludge is picked up in the pockets between the fluted surfaces. The sludge is progressively fed along between the surfaces with increasing pressure, thereby causing the water to be progressively removed through the screen. The solids form a cake or layer on the surface of the screen and are stripped therefrom by means of a stripping wire.
Similarly, U.S. Pat. No. 3,527,668, issued Sept. 8, 1970 to Kusters et al teaches an apparatus for removing water from cellulose webs by means of rolls contacting each other under pressure and between which the material passes. The roll peripheries are corrugated such that the rolls carry along the cellulose breadth and compress it therebetween.
A like device is shown in U.S. Pat. No. 4,475,453, issued Oct. 9, 1984 to Davis. The patent is directed to a method and apparatus for expressing the liquid phase from a wet mixture, such as a clay-like mixture. The apparatus disclosed includes a primary roll and a plurality of smaller pressure rolls having a thick outer covering of a deformable elastomeric material. A pair of opposed filter belts are trained about the primary roll for feeding the wet mixture through the nips formed between the primary and pressure rolls.
The prior art also discloses means for cutting diverse materials by feeding the material to be cut to a pair of rotating cutting members. Representative disclosures of devices of this type are shown in U.S. Pat. No. 3,529,777, issued Sept. 22, 1970 to Dodson et al and U.S. Pat. No. 4,374,573, issued Feb. 22, 1983 to Rouse et al.
U.S. Pat. No. 3,529,777 is directed to a process for granulating sheet-like material wherein the material to be granulated is fed into the bite of intermeshing teeth of a pair of rotating cutter members.
U.S. Pat. No. 4,374,573 discloses an apparatus for shredding waste material, such as rubber tires. The material to be shredded is fed to a pair of intermeshed cylindrical cutter rolls having a plurality of cutter discs.
It is also known in the prior art to utilize rotating rolls for crushing various substances. For instance, U.S. Pat. No. 1,691,546, issued Nov. 13, 1928 to Farrel, Jr., relates to a cane grinding apparatus comprising a pair of rotating toothed rolls which hook and draw in the cane so as to crush the cane and split open the stalks and separate the fibers.
U.S. Pat. No. 4,396,158, issued Aug. 2, 1983 to Olsen, discloses a device and method for processing hatchery offal by crushing or smashing it as it is passed between a pair of counterrotating rollers. The outer peripheries of the respective rollers are provided with a plurality of corrugations which mesh with, but do not contact, each other.
Similarly, U.S. Pat. No. 4,410,144, issued Oct. 18, 1983 to Imperi, is directed to a counterrotated crusher roll system wherein material to be crushed is fed into the nip between the teeth of a pair of power driven counterrotating rolls to be crushed into relatively fine particles. The teeth, which are formed on the exterior of the respective rolls, carry the material to be crushed downwardly through the nip and crush the material therein.
The aforementioned prior art fails to disclose a method of agglomerating and dewatering elastomers utilizing counterrotating pressing rolls capable of variable roll clearances and variable speed operation and being specifically adapted to process a material of the sort which undergoes continuous deformation.